The present invention relates to a pressure-sensitive adhesive composition comprising a crosslinked polymer obtained by crosslinking a block copolymer comprising at least two of a styrene-based polymer block A and an acrylic polymer block B block-bonded each other and a process for the preparation thereof. The present invention also relates to pressure-sensitive adhesive sheets of the pressure-sensitive adhesive composition in the form of sheet, tape or the like.
In recent years, pressure-sensitive adhesives such as solvent type pressure-sensitive, emulsion type pressure-sensitive adhesive and hot-melt type pressure-sensitive adhesive have been used for materials which are required to be easily adhered by simply pressing, such as packaging pressure-sensitive adhesive tapes, masking pressure-sensitive adhesive tapes for coating, sanitary pressure-sensitive adhesive tape, paper diaper fixing tape and pressure-sensitive adhesive label.
As the solvent type pressure-sensitive adhesives there have been known acrylic and rubber-based pressure-sensitive adhesives. In recent years, it has been required that the amount of pressure-sensitive adhesives to be used be minimized from the standpoint of drying efficiency, energy saving and working atmosphere. If the amount of the solvent to be used in the polymerization is reduced to meet this demand, a safety problem occurs due to difficulty in controlling the resulting polymerization heat. Further, the emulsion type pressure-sensitive adhesives are disadvantageous in that since they comprise polymer particles dispersed in water, the water content needs to be finally removed during the formation of the pressure-sensitive adhesive layer, resulting in the deterioration of drying efficiency and energy saving.
The hot-melt type pressure-sensitive adhesives are superior to the solvent type or emulsion type pressure-sensitive adhesives with respect to safety or economy. For example, hot-melt type pressure-sensitive adhesives mainly comprising styrene-isoprene block copolymer have been known. In general, however, this type of pressure-sensitive adhesives exhibits a poor light resistance and thus are disadvantageous in that the resulting products exhibit deterioration in properties with the lapse of time. In an attempt to overcome these difficulties and hence obtain pressure-sensitive adhesives free from these difficulties, acrylic polymer components, which are normally known to exhibit a good light resistance, are introduced instead of the isoprene-based polymer components, which cause the deterioration of the light resistance of the resulting products.
A random copolymer of acrylic monomer with styrene-based monomer can be easily synthesized. There are examples of an pressure-sensitive adhesive mainly comprising such a random copolymer. However, no products exhibiting satisfactory pressure-sensitive adhesive properties have been obtained. On the other hand, block copolymers of styrene-based polymer component and acrylic polymer component cannot be easily obtained by any of radical polymerization method, anionic polymerization method and cationic polymerization method. There are no examples of a pressure-sensitive adhesive mainly comprising such a block copolymer.
Accordingly, one object of the present invention is to provide a pressure-sensitive adhesive composition which comprises as an pressure-sensitive adhesive a block copolymer of a styrene-based polymer component and an acrylic polymer component that has been easily produced free from safety problems in the absence of solvent or in the presence of a small amount of a solvent to satisfy the desired pressure-sensitive adhesive properties in addition to the inherent characteristics due to the introduction of acrylic polymer component, i.e., enhancing the light resistance, without causing economic problems as in the conventional emulsion type pressure-sensitive adhesives, i.e., problems in drying efficiency and energy saving due to removal of water content.
Another object of the present invention is to provide a process for the preparation the pressure sensitive adhesive composition.
Still another object of the present invention is to provide pressure-sensitive adhesive sheets comprising the pressure-sensitive adhesive composition.
As a result of extensive studies on the above-described problems, it has been found that a living radical polymerization of a styrene-based monomer with an acrylic monomer in the presence of a specific activating agent and a polymerization initiator makes it easy to produce an A-B type or B-A type block copolymer or three-block or higher copolymers of styrene-based polymer block A and acrylic polymer block B, no appropriate synthesis methods of which having been known, in the absence of a solvent or in the presence of a small amount of a solvent without causing any problems in controlling the resulting polymerization heat. It has also been found that the use of a crosslinked polymer obtained by crosslinking the copolymer as a main component of a pressure-sensitive adhesive makes it possible to obtain a pressure-sensitive adhesive composition which sufficiently satisfies the desired pressure-sensitive adhesive properties, particularly well-balanced pressure-sensitive adhesive force and cohesive force and excellent heat resistance, in addition to the effect of enhancing the light resistance characteristic to the acrylic polymer block B without causing any economic problems as: in the conventional emulsion type pressure-sensitive adhesives. The present invention has been completed based on those findings.
The present invention provides a pressure-sensitive adhesive composition comprising a crosslinked polymer obtained by crosslinking a block copolymer comprising at least two of a styrene-based polymer block A and an acrylic polymer block B having a structural unit represented by the general formula (1): xe2x80x94[CH2xe2x80x94C(R1)COOR2]xe2x80x94 wherein R1 represents a hydrogen atom or methyl group, and R2 represents a C2-14 alkyl group), block-bonded each other.
The present invention also provides pressure-sensitive adhesive sheets comprising a layer of the pressure-sensitive adhesive composition having the above structure provided on a support.
The present invention further provides a process for the preparation of the pressure-sensitive adhesive composition, which comprises subjecting a styrene-based monomer and an acrylic monomer represented by the general formula (1A): CH2xe2x95x90C(R1)COOR2 wherein R1 represents a hydrogen atom or methyl group, and R2 represents a C2-14 alkyl group, optionally together with a monomer having an epoxy group in its molecule and/or a monomer having a hydroxyl group in its molecule, to a living radical polymerization in an appropriate order of monomers using a polymerization initiator in the presence of a transition metal and its ligand to produce a block copolymer comprising at least two of a styrene-based polymer block A and an acrylic polymer block B, block-bonded to each other, and then subjecting said block copolymer to crosslinking to produce a crosslinked polymer.
Details of the living radical polymerization method are described in various literature references, e.g., (1) Patten et al., xe2x80x9cRadical Polymerization Yielding Polymers with Mw/Mn xcx9c1.05 by Homogeneous Atom Transfer Radical Polymerizationxe2x80x9d, Polymer Preprinted, pp. 575-576, No. 37 (March 1996), (2) Matyjasewski et al., xe2x80x9cControlled/Living Radical Polymerization. Halogen Atom Transfer Radical Polymerization Promotedbya Cu(I)/Cu(II) Redox Processxe2x80x9d, Macromolecules 1995, 28, 7901-7910, Oct. 15, 1995, (3) PCT/US96/03302 to Matyjasewski et al., International Publication No. W096/30421, Oct. 3, 1996, (4) M. Sawamoto et al., xe2x80x9cRuthenium-mediated Living Radical Polymerization of Methyl Methacrylatexe2x80x9d, Macromolecules, 1996, 29, 1070.
The present inventors paid their attention to the living radical polymerization method. As a result, it was found that the living radical polymerization of a styrene-based polymer and an acrylic monomer in an appropriate order using a polymerization initiator in the presence of a transition metal and its ligand as an activating agent makes it easy to produce a block copolymer comprising at least two of styrene-based polymer block A and acrylic polymer block B, i.e., A-B type or B-A type block copolymer or three-block or higher block copolymers such as A-B-A type block copolymer.
Examples of the transition metal include Cu, Ru, Fe, Rh, V and Ni. In general, the transition metal used is selected from the group consisting of halides (chloride, bromide, etc.) of these metals. The ligand is coordinated with a transition metal as a center to form a complex. The ligandpreferably used is a bipyridine derivative, mercaptan derivative, trifluorate derivative or the like. Of the combinations of transition metal and its ligand, Cu+1-bipyridine complex is most preferable from the standpoint of polymerization stability or polymerization rate.
The polymerization initiator preferably used is an ester-based or styrene-based derivative containing a halogen in xcex1-position. In particular, a 2-bromo(or chloro)propionic acid derivative or chloro (or bromo)-1-phenyl derivative is more preferably used. Specific examples of these derivatives include methyl 2-bromo (or chloro)propionate, ethyl 2-bromo (or chloro)propionate, methyl 2-bromo(or chloro)-2-methylpropionate, ethyl 2-bromo or chloro)-2-methylpropionate and chloro(or bromo)-1-phenylethyl.
Examples of the styrene-based monomer to be used as one of the polymerizable monomers herein include styrene, xcex1-methylstyrene and 2,4-dimethylstyrene. The acrylic monomer to be used as the other one of the polymerizable monomers is an acrylic or methacrylic acid alkyl ester represented by the general formula (1A): CH2xe2x95x90CR1COOR2 wherein R1 represents a hydrogen atom or methyl group, and R2 represents a C2-14 alkyl group. In particular, (meth)acrylic acid alkyl ester having a C4-12 alkyl group, such as n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate and isononyl (meth)acrylate are preferably used.
As the acrylic monomer, a modifying monomer copolymerizable with the acrylic or methacrylic acid alkyl ester can be used in combination with the acrylic or methacrylic acid alkyl ester. In this case, the modifying monomer is used in an amount of 50% by weight or less, preferably 30% by weight or less, and more preferably 20% by weight or less, based on the total weight of the acrylic monomer in order to obtain good pressure-sensitive adhesive properties. Examples of the modifying monomer used include (meth)acrylamide, maleic acid monoester, maleic acid diester, glycidyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, N-vinylpyrrolidone, acrylonitrile and (meth)acryloylmorpholine.
In the living radical polymerization method, a styrene-based monomer is first polymerized. Subsequently, an acrylic monomer is added to continue the polymerization of monomers. Thus, an A-B type block copolymer can be produced. During this polymerization procedure, the acrylic monomer is added at the time when the amount of the styrene-based monomer added exceeds at least 50% by weight, normally 70% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more. On the other hand, if the acrylic monomer is polymerized prior to the addition and polymerization of the styrene-based monomer, a B-A type block copolymer can be produced. Similar to the above polymerization procedure, the styrene-based monomer is added at the time when the amount of the acrylic monomer added exceeds at least 50% by weight, normally 70% byweight or more, preferably 80% by weight ormore, and more preferably 90% by weight or more.
Further, if the living radical polymerization is carried out in amanner such thata styrene-basedmonomer is polymerized, an acrylic monomer is added to continue polymerization of monomers, and the styrene-based monomer is then added to continue polymerization of monomers, an A-B-A type block copolymer can be produced. During the successive polymerization procedure, the monomer to be subsequently added is added at the time when the conversion of the monomer which has been previously added exceeds at least 50% by weight, normally 60% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more.
Moreover, if the above three-stage polymerization is followed by the addition of the acrylic monomer to continue the polymerization of monomers, an A-B-A-B type block copolymer can be produced. If this polymerization procedure is then followed by the addition of the styrene-based monomer to continue the polymerization of monomers, an A-B-A-B-A type block copolymer can be produced. On the other hand, if an alternating living radical polymerization is effected in the same manner as described above except that the monomer to be first polymerized is changed to an acrylic monomer, a block copolymer such as B-A-B type, B-A-B-A type and B-A-B-A-B type block copolymers can be produced. In other words, the alternate living radical polymerization of a styrene-based monomer and an acrylic monomer makes it possible to produce various block copolymers comprising at least three of a styrene-based polymer block A and an acrylic polymer block B alternately bonded each other.
Two or more styrene-based polymer blocks A constituting the block copolymer comprising at least three blocks bonded each other may not be the same but may be styrene-based polymer blocks A1, A2 and A3 having different monomer compositions. Similarly, two or more acrylic polymer blocks B constituting the block copolymer may be acrylic polymer blocks B1, B2 and B3 having different monomer compositions.
In the present invention, it is generally preferred that a styrene-based monomer and an acrylic monomer be subjected to alternate living radical polymerization. However, when the styrene-based polymer blocks A (A1, A2, A3, etc.) or acrylic polymer blocks B (B1, B2, B3, etc.) have different monomer compositions which are definitely distinguished from each other in properties, the order of monomers to be subjected to living radical polymerization may be changed as necessary to produce three-block or higher block copolymers which do not necessarily comprise a styrene-based polymer block A and an acrylic polymer block B alternately bonded each other, such as A1-A2-B type, B1-B2-A type, A1-A2-B-A3 type, B1-B2-B3 type, A1-B-A2-A3 type, B1-A-B2-B3 type and A1-B1-A2-B2 type block copolymers.
In the living radical polymerization process, the polymerization initiator may be used in an amount of normally from 0.01 to 10 mol %, preferably from 0.1 to 5 mol %, and more preferably from 0.1 to 2 mol %, per mole of the sum of the polymerizable monomers containing a styrene-based monomer and an acrylic monomer (if a monomer containing a hydroxyl group or epoxy group in its molecular as described later is used, the sum of polymerizable monomers containing these monomers is used). The transition metal is used in the form of halide or the like in an amount of normally from 0.01 to 3 mols, and preferably from 0.1 to 1 mol, per mole of the polymerization initiator. The ligand of the transition metal is used in an amount of normally from 1 to 5 mols, and preferably from 2 to 3 mols, per mole of the transition metal which may be in the form of halide. The use of the polymerization initiator and the activating agent in the above defined proportion makes it possible to provide good results in the reactivity of living radical polymerization and the molecular weight of the resulting polymer.
The living radical polymerization can be proceeded without solvent or in the presence of a solvent such as butyl acetate, toluene and xylene. If the solvent is used, it is used in a small amount such that the solvent concentration after polymerization is 50% by weight or less in order to prevent the drop of polymerization rate. Even if the living radical polymerization is effected free from solvent or in the presence of a small amount of a solvent, little or no safety problems concerning the control over polymerization heat can occur. Rather, reduction in the amount of solvent used makes it possible to provide good results in economy, environmental protection, etc. Referring to the polymerization conditions, the living radical polymerization is carried out at a temperature of from 70xc2x0 C. to 130xc2x0 C. for about 1 to 100 hours, though depending the final molecular weight or polymerization temperature, taking into account the polymerization rate or deactivation of catalyst.
The block copolymer thus produced, if it is of A-B type, has a structure comprising a styrene-basedi polymer block A as a starting point having an acrylic polymer block B having a structural unit represented by the general formula (1): xe2x80x94[CH2xe2x95x90C(R1)COOR2]xe2x80x94 wherein R1 represents a hydrogen atom or methyl group, and R2 represents a C2-14 alkyl group, bonded thereto. If it is of B-A type, the block copolymer has a structure comprising the above acrylic polymer block B as a starting point having the styrene-based polymer block A bonded thereto. If it is of A-B-A type, the block copolymer has a structure comprising a styrene-based polymer block A as a starting point having the above acrylic polymer block B and styrene-based polymer block A sequentially bonded thereto. If it is of B-A-B type, the block copolymer has a structure comprising the above acrylic polymer block B as a starting point having a styrene-based polymer block A and an acrylic polymer block B sequentially bonded thereto. The block copolymer comprising at least two blocks connected to each other has a microdomain structure as in widely used styrene-isoprene-styrene block copolymers. It is presumed that this microdomain structure allows the block copolymer to exhibit well-balanced pressure-sensitive adhesive force and cohesive force when used as a pressure-sensitive adhesive.
The block copolymer comprising at least two blocks bonded each other comprises a styrene-based polymer block in a proportion not exceeding 50% by weight, preferably not exceeding 40% by weight, and more preferably 5 to 20% by weight, based on the total weight of the copolymer if it is of A-B or B-A type, or in a proportion of not exceeding 60% by weight, and preferably from 5 to 40% by weight, based on the total weight of the copolymer if it is three-block type such as A-B-A and B-A-B. If the proportion of the styrene-based polymer block A is too large, the resulting polymer lacks required viscoelasticity and thus is too hard for pressure-sensitive adhesives, which is not preferable. On the other hand, if the proportion of the styrene-based polymer block A is too small, the resulting polymer lacks cohesive force required for pressure-sensitive adhesives, which is also not preferable.
The present invention may optionally use, as the polymerizable monomer, a monomer containing an epoxy group or hydroxyl group in its molecule besides the styrene-based monomer and acrylic monomer. In this case, the structural unit derived from these monomers is contained in either the styrene-based polymer block A or the acrylic polymer block B depending on the time at which these monomers are added. Accordingly, the term xe2x80x9ctotal weight of the block copolymerxe2x80x9d as used herein means to indicate the sum of the weight of the styrene-based polymer block A and the acrylic polymer block B. However, the blocks A and B each have a structural unit derived from the above monomer containing a hydroxyl group or epoxy group in its molecule.
In the present invention, the block copolymer comprising at least two blocks bonded each other has a number average molecular weight of normally from 5,000 to 500,000, and preferably from 10,000 to 200,000, from the standpoint of pressure-sensitive adhesive properties and coatability. The term xe2x80x9cnumber average molecular weightxe2x80x9d as used herein means to indicate value determined by GPC (gel permeation chromatography) method in polystyrene equivalence.
The block copolymer preferably has a proper functional group in its polymer chain to facilitate its crosslinking at the final step. The kind of the functional group used is appropriately selected depending on the crosslinking method. For example, if the crosslinking treatment is effected with a polyfunctional isocyanate as a crosslinking agent under heating, the functional group reactive with the crosslinking agent is preferably a hydroxyl group. Further, in order to solve the problems concerning the control over the reaction time, i.e., pot life, by the use of the polyfunctional isocyanate, the functional group in the polymer chain, if the epoxy-crosslinking treatment is effected, is preferably an epoxy group or hydroxyl group.
The block copolymer having a hydroxyl group in its polymer chain suitable for crosslinking can be easily produced by using a material containing a hydroxyl group in its molecule as a polymerization initiator and/or using a monomer containing a hydroxyl group in its molecule as one of the polymerizable monomers.
The use of the polymerization initiator containing a hydroxyl group in its molecule makes it possible to introduce the hydroxyl group into the starting end of the polymer chain. Such a polymerization initiator used is an ester-based or styrene-based derivative containing a halogen in a-position and having a hydroxyl group in its molecule. Specific examples of the derivative used include 2-hydroxyethyl 2-bromo(or chloro)propionate, 4-hydroxybutyl 2-bromo(or chloro)propionate, 2-hydroxyethyl 2-bromo(or chloro)-2-methylpropionate, and 4-hydroxybutyl 2-bromo(or chloro)-2-methylpropionate. The polymerization initiator having a hydroxyl group in its molecule may be used in combination with the above polymerization initiator having no hydroxyl group in its molecule, with the proviso that the sum of the amount of the two polymerization initiators is as defined above.
If a monomer having a hydroxyl group in its molecule is used, the hydroxyl group can be introduced into the polymer chain at an arbitrary position depending on the time at which the monomer is added. Such a monomer used is an acrylic or methacrylic acid hydroxyalkylester represented by the general formula (2A): CH2xe2x95x90CR3COOR4 wherein R3 represents a hydrogen atom or methyl group, and R4 represents a C2-6 alkyl group having at least one hydroxyl group. Specific examples of the acrylic or methacrylic acid hydroxyalkylester include 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth) acrylate and 6-hydroxyhexyl (meth) acrylate. Such a monomer is used in an amount of 10% by weight or less, and preferably 5% by weight or less, based on the total weight of the polymerizable monomers in order to maintain good pressure-sensitive adhesive properties.
The combined use of a polymerization initiator having a hydroxyl group in its molecule and a monomer having a hydroxyl group in its molecule makes it possible to provide better results in pressure-sensitive adhesive properties after crosslinking. In particular, if the monomer is added in the late stage of polymerization, i.e., at the time when the conversionof polymer reaches 80% by weight during the formation of the final stage polymer block (e.g., second stage for A-B or B-A type, third stage for A-B-A or B-A-B type), the hydroxyl group of the monomer can be introduced into the polymer chain at its terminal, in combination with the hydroxyl group derived from the polymerization initiator introduced into the polymer chain at its starting end. Thus, two or more hydroxyl groups are telechelically introduced into the block copolymer. As a result, the crosslinking reaction causes the polymer to extend linearly, making it possible to obtain a uniform crosslinked polymer having a small dispersion of interbridge distance that brings about good results in the enhancement of pressure-sensitive adhesive properties.
Examples of the block copolymer having an epoxy group or hydroxyl group in its polymer chain suitable for epoxy crosslinking include (a) block copolymer containing at least two epoxy groups per molecule, (b) block copolymer containing at least one epoxy group and at least one hydroxyl group per molecule and (c) block copolymer containing at least two hydroxyl groups per molecule.
The block copolymer (a) preferably:contains an epoxy group incorporated therein at or in the vicinity of the end of molecular chain. The block copolymercanbeeasilysynthesized by using a monomer having an epoxy group in its molecule as a monomer other than the styrene-based or acrylic monomer with a polymerization initiator having an epoxy group in its molecule.
If the monomer having an epoxy group in its molecule is used in the living radical polymerization, process, the epoxy group can be introduced into the polymer chain at an arbitrary position depending on the time at which the monomer is added. Accordingly, when the monomer is added in the late stage of polymerization, i.e., at the time when the conversion of styrene-basedmonomerandacrylicmonomer reaches 80% byweight, an epoxy group can be introduced into the polymer chain at or in the vicinity of the terminal thereof. If the polymerization reaction is effected in the presence of a polymerization initiator having two starting points per molecule, two epoxy groups are telechelically introduced into the molecular chain of copolymer. Alternatively, by adding the monomer separately, i.e., in the initial stage of polymerization and the late stage of polymerization, so that an epoxy group is introduced into the polymer chain at or in the vicinity of starting end of the polymer chain and at or in the vicinity of terminal of the polymer chain, the same telechelic structure as described above can be obtained. When such a block copolymer is epoxy-crosslinked to cure, the molecular chain of copolymer can extend linearly, making it possible to produce a uniform a crosslinked polymer having a small dispersion of interbridge distance that provides good results in the enhancement of pressure-sensitive adhesive properties.
The monomer having an epoxy group in its molecule is represented by the general formula (3A): CH2xe2x95x90C(R5)COOR6 wherein R5 represents a hydrogen atom or methyl group, and R6 represents an alkyl group containing an epoxy group. Specific examples of the monomer include glycidyl (meth)acrylate, methylglycidyl (meth) acrylate, 3,4-epoxycyclohexylmethyl (meth) acrylate and 6-methyl-3,4-epoxycyclohexylmethyl (meth)acrylate. The amount of such a monomer to be used is normally 40% by weight or less, and preferably 4% by weight or less, based on the total weight of the polymerizable monomers in order to maintain good pressure-sensitive adhesive properties.
Further, the polymerization in the presence of the polymerization initiator having an epoxy group in its molecule makes it possible to introduce an epoxy group into the polymer chain at the starting end thereof. Accordingly, if an epoxy group is introduced into the polymer chain at the starting end thereof by using the polymerization initiator having an epoxy group in its molecule while introducing an epoxy group into the polymer chain at or in the vicinity of the terminal thereof by adding the monomer having an epoxy group in its molecule at the late stage of polymerization, two epoxy groups are telechelically introduced into the molecular chain of the copolymer. As a result, when the block copolymer thus obtained is then epoxy-crosslinked to cure, the molecular chain of the copolymer extends more linearly to produce a uniform crosslinked polymer having a small dispersion of interbridge distance that provides good results in the enhancement of pressure-sensitive adhesive properties.
The polymerization initiator having an epoxy group in its molecule used can be any ester-based or styrene-based derivative having a halogen in xcex1-position and an epoxy group in its molecule so long as it does not inhibit the progress of living radical polymerization. Specific examples of such an ester-based or styrene-based derivative used include glycidyl 2-bromo(or chloro)propionate, glycidyl 2-bromo(or chloro)-2-methylpropionate, 3,4-epoxycyclohexylmethyl 2-bromo(or chloro)propionate and 3,4-epoxycyclohexylmethyl 2-bromo(or chloro)-2-methylpropionate.
The block copolymer (b) preferably comprises an epoxy group incorporated therein at or in the vicinity of one end of the molecular chain and a hydroxyl group incorporated therein at or in the vicinity of the other end of the molecular chain.
Such a block copolymer can be easily synthesized by (1) using as monomers other than the styrene-based monomer and acrylic monomer a monomer having an epoxy group in its molecule and a monomer having a hydroxyl group in its molecule in combination or (2) using a polymerization initiator having a hydroxyl group in its molecule together with the monomer having an epoxy group in its molecule or (3) using the monomer having a hydroxyl group in its molecule together with the polymerization initiator having an epoxy group in its molecule.
In accordance with the method (1), a monomer having an epoxy group in its molecule is added in the initial stage of polymerization, and a monomer having a hydroxyl group in its molecule is then added in the late stage of polymerization. Alternatively, the monomer having a hydroxyl group in its molecule is added in the initial stage of polymerization, and the monomer having an epoxy group in its molecule is then added in the late stage of polymerization. In this manner, an epoxy group (or hydroxyl group) can be introduced into the polymer chain at or in the vicinity of the starting end thereof while a hydroxyl group (or epoxy group) can be introduced into the polymer chain at or in the vicinity of the terminal thereof. Thus, an epoxy group and a hydroxyl group are telechelically introduced into the molecular chain of the copolymer. As a result, when the block copolymer thus obtained is then crosslinked between the epoxy groups or between the epoxy group and the hydroxyl group to cure, the molecular chain of the copolymer extends more linearly to produce a uniform crosslinked polymer having a small dispersion of interbridge distance that provides good results in pressure-sensitive adhesive properties.
In accordance with the method (2), a hydroxyl group is introduced into the polymer chain at the starting end thereof by using a polymerization initiator having a hydroxyl group in its molecule, and an epoxy group is then introduced into the polymer chain at or in the vicinity of the terminal thereof by adding a monomer having an epoxy group in its molecule in the late stage of polymerization. In this manner, an epoxy group and a hydroxyl group are telechelically introduced into the molecular chain of the copolymer. Similarly, in accordance with the method (3), an epoxy group is introduced into the polymer chain at the starting end thereof by using a polymerization initiator having an epoxy group in its molecule, and a hydroxyl group is then introduced into the polymer chain at or in the vicinity of the terminal thereof by adding a monomer having a hydroxyl group in its molecule in the late stage of polymerization. In this manner, an epoxy group and a hydroxyl group are similarly telechelically introduced into the molecular chain of the copolymer. Similarly, when the block copolymer thus obtained is then crosslinked between the epoxy groups or between the epoxy group and the hydroxyl group to cure, the molecular chain of the copolymer extends more linearly to produce a uniform crosslinked polymer having a small dispersion of interbridge distance that provides good results in pressure-sensitive adhesive properties.
The block copolymer (c) preferably comprises a hydroxyl group incorporated therein at or in the vicinity of the molecular chain. The block copolymer can be easily synthesized by using, as a monomer other than the styrene-based monomer and acrylic monomer, a monomer having a hydrbxyl group in its molecule, or using such a monomer together with a polymerization initiator having a hydroxyl group in its molecule.
A monomer having a hydroxyl group in its molecule is added in the late stage of polymerization so that a hydroxyl group is introduced into the polymer chain at or in the vicinity of the terminal thereof, during which a polymerization initiator having two starting points per molecule is used. Alternatively, the monomer is added separately in the initial stage of polymerization and in the late stage of polymerization so that a hydroxyl group is introduced into the polymer chain at or in the vicinity of the starting end thereof and at or in the vicinity of the terminal end thereof. Alternatively, the monomer having a hydroxyl group in its molecule is added in the late stage of polymerization so that a hydroxyl group is introduced into the polymer chain at or in the vicinity of the terminal thereof, during which a polymerization initiator having a hydroxyl group in its molecule is used so that a hydroxyl group is introduced into the polymer chain at the starting end thereof. In this manner, a block copolymer comprising two hydroxyl groups telechelically incorporated in its molecular chain can be synthesized. When the block is then crosslinked with an epoxy-crosslinking agent so that the epoxy group in the crosslinking agent and the hydroxyl group in the copolymer are crosslinked with each other, the molecular chain of the copolymer extends more linearly to produce a uniform crosslinked polymer having a small dispersion of interbridge distance that provides good results in pressure-sensitive adhesive properties.
In the present invention, the block copolymer is crosslinked to cause the extension of the main chain and the network formation at the same time, thereby producing a crosslinked polymer having a long molecular chain. The use of the crosslinked polymer as a main component of pressure-sensitive adhesive makes it possible to obtain an pressure-sensitive adhesive composition which remarkably satisfies the desired pressure-sensitive adhesive properties, particularly well-balanced pressure-sensitive adhesive peeling force and cohesive force and excellent heat resistance. The crosslinking method is not specifically limited. Various conventional crosslinking methods can be employed. One of the effective methods, if the block copolymer contains a hydroxyl group incorporated in the polymer chain, comprises heating the block copolymer with a polyfunctional isocyanate incorporated therein as a crosslinking agent so that the hydroxyl group in the block copolymer reacts with the isocyanate group as previously described.
Examples of the polyfunctional isocyanate used include tolylene diisocyanate, diphenylmethane diisocyanate, p-phenylene diisocyanate, hexamethylene diisocyanate, 1,5-napthalene diisocyanate, adducts of these diisocyanates with polyvalent alcohols such as propanetriol, and tricyanurate derivatives obtained by trimerizing these diisocyanates. These polyfunctional isocyanates may be heated during crosslinking in the form of block, particularly in the form of compound protected by ethyl acetoacetate, methyl ethyl ketoxime, caprolactam or the like, so that it is activated before use.
The amount of the polyfunctional isocyanate to be used depends on the number of hydroxyl groups contained in the block copolymer. In practice, however, the polyfunctional isocyanate is preferably used in an amount of from 0.05 to 5 parts by weight per 100 parts by weight of the block copolymer. If the amount of the polyfunctional isocyanate exceeds the above defined range, the resulting pressure-sensitive adhesive force is reduced. On the other hand, if the amount of the polyfunctional isocyanate falls below the above defined range, the resulting cohesive force is insufficient. The crosslinking treatment may be effected by heating to a temperature of from 50 to 150xc2x0 C. The crosslinking treatment may be effected in the presence of a catalyst such as tin compound to increase the crosslinking rate.
Another crosslinking method, if the block copolymer contains an epoxy group in the polymer chain, particularly one belonging to the block copolymers (a) to (c), comprises subjecting the block copolymer to irradiation with ultraviolet rays in the presence of an onium salt-based curing catalyst and optionally an epoxy-based crosslinking agent so that it is epoxy-crosslinked. This method is advantageous in that it requires reduced energy, can be effected at a high efficiency and requires no heat-resistant support (i.e., object to which this method is applied is not limited) as compared with the heating method using a polyfunctional isocyanate.
The epoxy-based crosslinking agent used is a compound having two or more epoxy groups per molecule. Examples of such a compound include ethylene glycol diglycidyl ether (hereinafter referred to as xe2x80x9cEGDxe2x80x9d), glycerin diglycidyl ether, vinyl cyclohexene dioxide represented by the general formula (El) shown later, limonene dioxide represented by the general formula (E2) shown later, 3,4-epoxycyclohexylmethyl-3xe2x80x2, 4xe2x80x2-epoxycyclohexyl carboxylate (hereinafter referred to as xe2x80x9cBEPxe2x80x2) represented by the general formula (E3) shown later, bis-(3,4-epoxycyclohexyl)adipate represented by the general formula (E4) shown later, trifunctional epoxy compound (hereinafter referred to as xe2x80x9c3EPxe2x80x9d) represented by the general formula (E5) shown later, and tetrafunctional epoxy compound (hereinafter referred to as xe2x80x9c4EPxe2x80x9d) represented by the general formula (E6) shown later.
These epoxy-based crosslinking agents are not essential components for epoxy crosslinking and thus may be or may not be used if the block copolymer is one belonging to the block copolymers (a) and (b) because the block copolymer has an epoxy group in its polymer chain. On the other hand, the block copolymer (c) has no epoxy group in its polymer chain and thus cannot be epoxy-crosslinked without such an epoxy-based crosslinking agent. The amount of such an epoxy-based crosslinking agent, if used, is normally 50 parts by weight or less, and preferably 30 parts by weight or less, per 100 parts by weight of the block copolymer in order to obtain good pressure-sensitive adhesive properties. 
wherein a+b=1, and Z is 3,4-epoxycyclohexyl group represented by the following general formula: 
wherein a+b+c+d=3, and Z is 3,4-epoxycyclohexyl group represented by the following general formula: 
The onium salt-based curing catalyst used is preferably a diazonium salt, sulfonium salt or iodonium salt represented by ArN2+Qxe2x88x92, Y3S+Qxe2x88x92 or Y2I+Qxe2x88x92, respectively, wherein Ar represents an aryl group such as bis (dodecylphenyl), Y represents an alkyl group or an aryl group defined above, and Qxe2x88x92 represents a nonbasic nucleophilic anion such as BF4xe2x88x92, PF6xe2x88x92, AsF6xe2x88x92, SbF6xe2x88x92, SbCl6xe2x88x92, HSO4xe2x88x92 and Cl.
Specific examples of the onium salt-based curing catalyst usedincludebis(dodecylphenyl) iodoniumhexafluoroantimonate, bis(t-butylphenyl)iodonium hexafluorophosphate, bis(t-butylphenyl)iodonium trifluoromethanesulfonate, triphenylsulfonium trifluoromethanesulfonate, biphenyliodonium trifluoromethanesulfonate, phenyl-(3-hydroxy-pentadecylphenyl)iodonium hexafluoroantimonate, diaryliodoniumtetrakis(pentafluorophenyl)borate, and compounds containing these components. Besides these compounds, various mixtures containing the above components, e.g., UV-9380C, produced by Toshiba Silicone Co., Ltd., a product containing 45% by weight of bis (dodecylphenyl) iodonium hexafluoroantimonate, can be used. The amount of such an onium salt-based curing catalyst to be used is normally from 0.01 to 20 parts by weight, and preferably from 0.1 to 5 parts by weight, per 100 parts by weight of the block copolymer. If the amount of the onium salt-based curing catalyst is too small, the curability by crosslinking reaction is poor. On the other hand, if the amount of the onium salt-based curing catalyst is too large, the pressure-sensitive adhesive properties deteriorate.
The process involving the irradiation with ultraviolet rays in the presence of such an onium salt-based curing catalyst can be carried out by using an appropriate ultraviolet light source such as high-pressure mercury lamp, low-pressure mercury lamp and metal halide lamp. The exposed dose is not specifically limited. In practice, however, it is normally from 50 mJ to 5 J/cm2. During this procedure, a filter or polyester sheet which cuts ultraviolet rays at the short wave side may be used. The irradiation temperature is not specifically limited. In practice, however, it can normally range from room temperature to 120xc2x0 C.
The pressure-sensitive adhesive composition of the present invention may comprise a crosslinked polymer obtained by crosslinking as described above and the block copolymer comprising at least two of styrene-based polymer block A and acrylic polymer block B bonded each other as a main component and optionally various additives which are incorporated in conventional pressure-sensitive adhesive compositions, such as tackifying resins, fillers, antioxidants and pigments.
The pressure-sensitive adhesive sheets of the present invention are obtained by a process which comprises applying an uncrosslinked pressure-sensitive adhesive composition of the present invention to one or both surfaces of a support, optionally drying the coated material, and then subjecting the coated material to crosslinking in the same manner as described above to form a layer of the pressure-sensitive adhesive composition normally having a thickness of from 10 to 100 xcexcm on each side, thereby producing a tape or sheet form. The support used is papers, plastic-laminated papers, cloth, plastic-laminated cloth, plastic film, metal foil, foamed products or the like. Applying the pressure-sensitive adhesive composition to the support can be accomplished by means of a hot melt coater, comma roll, gravure coater, roll coater, kiss coater, slot die coater, squeeze coater or the like.